1. Field of the Invention
This invention relates to propylene-ethylene copolymers for high-rigidity molded products and a process for producing the same. More particularly it relates to the copolymers from which molded products having high rigidity and superior high-impact properties at the same time are obtained even without adding any particular additive, and a process for producing the same.
2. Description of the Prior Art
Crystalline polypropylene which will hereinafter be often referred to as polypropylene, as a general-purpose resin, exhibits a high rigidity, hardness, tensile strength, heat-resistance, etc. However, it has such a drawback that it is insufficient in high-impact properties so that its use for molded products which are to be subjected to mechanical impact or used at low temperatures is difficult.
In general, the rigidity, hardness, heat-resistance, etc. of plastic materials have an inconsistent relationship with the high-impact properties thereof, and it often enough is very difficult to improve the formers and the latter at the same time. In order to broaden concrete application fields and hence demand of polypropylene, it is desired to improve not only the high-impact properties, but also the rigidity much more. If these physical properties are improved, it becomes possible to use polypropylene in the application fields where other general-purpose resins such as ABS resin, high-impact polystyrene, etc. have been used.
A number of proposals for improving the high-impact properties of polypropylene have been made. For example, an elastomer represented by ethylene-propylene rubber i.e. EPR is blended with polypropylene. In this case, however, the dispersion of the elastomer is nonuniform as compared with block copolymers so that the weld strength and glossiness of molded products are reduced and also the rigidity is reduced. Further, blending of EPR makes the cost of polypropylene composition higher. Another process wherein propylene is random- or block-copolymerized with another .alpha.-olefin such as ethylene for the same purpose as the above-mentioned has been known. The resulting random copolymers are insufficient particularly in the improvement of low-temperature high-impact properties as compared with polypropylene, and as the ethylene content is increased, the rigidity, strength, heat-resistance, etc. lower rapidly. On the other hand, the block copolymers have notably improved values of low-temperature high-impact properties as compared with polypropylene, whereas the rigidity, strength, heat-resistance, etc. are reduced. In order to overcome the above-mentioned drawbacks of the block copolymerization process, a number of processes have been proposed. For example, Japanese patent application laid-open Nos. Sho 50-115296/1975, Sho 52-4588/1977 and Sho 53-35879/1978 disclose a process of carrying out propylene-ethylene block copolymerization at a plurality of stages. Further, Japanese patent publication Nos. Sho 47-8207/1972, Sho 49-13231/1974, Sho 49-13231/1974 and Sho 49-13514/1974 disclose an improved process of adding a third component to catalyst. Still further, Japanese patent application laid-open Nos. Sho 55-764/1980, Sho 54-152095/1979 and Sho 53-29390/1978 and Japanese patent publication No. Sho 55-8011/1980 disclose an improved process of using a specified catalyst. These proposed processes, however, are directed to a technique for alleviation, of making the degree of reduction in the rigidity of the block copolymers as small as possible as compared with polypropylene (homopolymer); thus it has not yet been possible for the block copolymers to have a rigidity value on the same level as or a higher level than that of the homopolymer.
In view of the present status of the known art, the present inventors have made strenuous studies on a propylene-ethylene block copolymer from which molded products having superior high-impact properties and a high rigidity can be obtained without adding any particular additive, as well as a process for producing the same, and as a result have found that such molded products having superior high-impact properties and a high rigidity can be obtained for the first time by using a polypropylene prepared under the defined conditions of the present invention as described below. As apparent from the foregoing, an object of the present invention is to provide a process for producing a propylene-ethylene block copolymer suitable for molding it into molded products having superior high-impact properties and a high rigidity at the same time. Another object of the present invention is to provide a propylene-ethylene block copolymer for high-rigidity molded products.